Highly pathogenic simian-human immunodeficiency viruses (SHIVs) cause a rapid, systemic, and complete loss of CD4+ T lymphocytes within weeks of inoculation of rhesus monkeys, high and sustained levels (>107 copies/ml plasma) of viral RNA, and death from immunodeficiency in 12 to 30 weeks (1-3). This rapid and irreversible CD4+ T cell depleting phenotype is markedly different from that observed for either simian immunodeficiency virus (SIV) or HIV-1, both of which generally induce more gradual and moderate losses of CD4+ T cells and much slower development of clinical disease in macaques and humans, respectively. The distinctive features of in vivo SHIV and SIV infections suggest that the underlying mechanism(s) of disease induction by each virus may be different. Side-by-side comparisons of CD4+ T cell subsets targeted by SHIVs and SIV during infections of rhesus monkeys were therefore carried out. In the case of SIV, four (of the 10 initially inoculated) rapidly progressing animals were studied because they exhibit virological parameters and clinical symptoms that are similar to those reported for macaques infected with SHIVs. Ex vivo experiments, employing competitors specific for CCR5 and CXCR4, revealed that SIVs exclusively used CCR5 to enter and spread through cultured rhesus monkey PBMC whereas SHIV-DH12R utilized CXCR4, not CCR5, during productive infections of the same cells. In inoculated rhesus monkeys, massive depletion of naive CD4+ T lymphocytes was observed during the period of peak SHIV production (weeks 1 to 3). In contrast, memory CD4+ T cells were selectively eliminated from the blood and lymph nodes in rapidly progressing SIV infected animals. The expression of CXCR4 on virtually all naive and a significant fraction of memory CD4+ T lymphocytes ultimately makes both subsets the target of highly pathogenic SHIVs, and explains the rapid, irreversible, and complete depletion of CD4+ T cells in SHIV but not SIV infections of macaques. SIV/HIV chimeric viruses (SHIVs), carrying several HIV-1 genes, have been constructed and used to monitor the roles of the HIV-1 envelope glycoprotein in infected animals. Highly pathogenic SHIVs have recently been obtained, which induce a rapid (within 2 to 3 weeks of inoculation), irreversible depletion of CD4+ T lymphocytes in the peripheral blood of acutely infected rhesus monkeys. Infected animals survive an additional 4 to 6 months and high levels of viremia are sustained in the absence of CD4+ T lymphocytes by macrophage in lymphoid tissues and the gastrointestinal tract. We have investigated whether specific changes in the viral genome accompany the transition from the T cell to the macrophage phase of SHIV infected macaques. Viral variants, circulating in the plasma during the macrophage phase of SHIV infections, carried gp120 V2 changes (specific double amino acid deletions and the loss of a conserved glycosylation site). Some of these SHIV variants had also acquired the capacity to infect alveolar macrophage (AM). SHIVs, isolated directly from lymphoid tissues, bore envelope glycoproteins that were genetically more heterogeneous than strains recovered from plasma and contained specific changes within V1 and V2. Small molecule coreceptor-targeted inhibitors, specific for CCR5 or CXCR4, were used to assess the chemokine receptor usage by: 1) the starting highly pathogenic, T-cell deleting, SHIV and, 2) late phase M-tropic SHIV variants. Blockade of CXCR4 potently suppressed infection of rhesus monkey PBMC by both viruses, whereas CCR5 targeted inhibitors had little if any effect. Infection of macaque AM by M-tropic SHIVs was also suppressed by CXCR4, not CCR5 specific inhibitors. Thus, the acquisition of macrophage tropism by SHIVs, present in late phase monkeys, is not accompanied by a switch in chemokine coreceptor usage.